PROJECT SUMMARY/ABSTRACT National surveys show that among U.S. adolescents, e-cigarettes are the most commonly used nicotine or tobacco product. Research also shows that adolescent e-cigarette users? are susceptible to transitioning to combustible tobacco use or dual use (use of e-cigarettes and combustible tobacco), increasing their risk for nicotine dependence. It critical that we better understand the risk factors and health effects associated with e-cigarette use, including the transition to combustible tobacco products and the differential risk for e-cigarette only users and dual users, in order to inform FDA regulations. Research suggests that socio-ecological factors, such as e-cigarette access, perceptions, marketing, and use by others, may lead to a greater susceptibility to future willingness and use of combustible tobacco products. However, extant research is insufficient due to the use of retrospective measures, largely cross-sectional methods, and the focus on between-persons designs. The proposed study will be one of the first to use a longitudinal ecological momentary assessment (EMA) approach and baseline CASI survey to examine both within-person and between-persons associations of e-cigarette and tobacco use, intentions, and willingness, and the role of environment and situational factors on adolescents? use of e- cigarettes and tobacco. Data will be collected from 50 adolescents (ages 13-17), who are e-cigarette only users or dual users of e-cigarettes and tobacco, and living in Kentucky, a state where e-cigarette use rates are three times higher than the national average for middle schoolers and two times higher than the national average for high schoolers. Using a framework based in the Social Ecological and Prototype Willingness Models, the specific aims for the proposed study are: (1) to investigate within-person associations among e-cigarette and tobacco access, motivations, exposure, context, intentions, willingness, and behaviors; (2) to investigate between-person associations among e-cigarette and tobacco beliefs, norms, access, motivations, exposure, context, intentions, willingness, and behaviors; and (3) to explore differences between e-cigarette only users and dual users on e-cigarette and tobacco beliefs, access, motivations, exposure, context, intentions, willingness, and behaviors as well as differences among the two groups on personal risk factors. The results of this study will provide strong preliminary data, including estimates of effect sizes, to guide the design of the larger, more rigorous R01 study to address significant shortcomings in the literature and inform FDA?s Center for Tobacco Products (CTP) research priorities: (#4) e-cigarette initiation, use, and perceptions, and transitions to tobacco products and multiple use, (#6) the impact of tobacco product characteristics, (e.g., flavors, labeling and marketing) on initiation and use, and (#9) perceptions of tobacco products including the impact of labeling and marketing. In line with the goals of RFA OD 15 004, this study builds on successful collaboration of a promising New Investigator with experienced scientists in tobacco and e-cigarette research for preparation of an R01 that will launch an independent career in tobacco research.